A mechanical gearbox comprises a primary shaft which receives motion from a crankshaft of a thermal engine of the vehicle by means of the interposition of a clutch, and a secondary shaft which transmits the motion to the drive wheels of the vehicle. The primary shaft and the secondary shaft are reciprocally and mechanically coupled by means of a plurality of gear pairs, each of which defines a corresponding speed and a comprises a primary gear mounted on the primary shaft and a secondary gear mounted on the secondary shaft.
According to a first possible embodiment, in each gear pair which defines a corresponding speed, the primary gear permanently meshes with the corresponding secondary gear; furthermore, in each gear pair, one gear is rigidly mounted onto its shaft, while the other gear is idly mounted on its shaft and coupled with a synchronizer adapted to be actuated to engage the other gear itself to its shaft (i.e. to make the gear integral with the shaft).
In order to reduce the number of gearbox components, each synchronizer is coaxially mounted with respect to a shaft between two gears so as either to be displaced in one sense to engage a gear onto the shaft or to be displaced in the other sense to engage the other gear onto the shaft. A plurality of forks are provided in order to actuate the synchronizers, which forks envelop a corresponding synchronizer to alternatively displace the synchronizer itself in the two senses along the shaft.
During a gear shift, once the clutch has been opened to disconnect the primary shaft from the crankshaft, the synchronizer of the gear of the current speed is actuated to disengage the gear of the current speed from the shaft, and thus the synchronizer of the gear of the subsequent speed is actuated to engage the gear of the subsequent speed onto the shaft. In a currently marketed mechanical gearbox, the gear pairs corresponding to consecutive speeds are arranged reciprocally side-by-side and share a same synchronizer; thus frequently a same synchronizer must be activated both to disengage the gear of the current speed from the shaft and to engage the gear of the subsequent speed onto the shaft. However, in this case, the gear shift is relatively slow because before starting to engage the gear of the subsequent speed onto the shaft it is necessary to wait for the gear of the current speed to be fully disengaged from the shaft.